I modded my door key fob that is used to enter our building.
Since a couple of weeks, we have new keys for our university buildings, namely an RFID keyring fob. However, they are rather ugly, and by the philosophy that we should strive for beauty, I decided to do something about it!

First, the results

Left 'before', an ugly black/gray, plastic, hard thingy. Not a good way to start your day. On the right, a nice soft pink/blue elephant. Now that will surely cheer you up! (Pictures by Erwin.)

The process

The original keyfob, with the original utensil (courtesy Erwin again) to open it. The trick here is the old saying: "If brute force doesn't help, you aren't using enough of it.
The fob has been opened! To the right the black container, to the left the gray center. (I already 'processed' the gray part somewhat, so it is a little damaged, see below.) I was expecting some kind of circular battery, but it is much more ingenious. The RFID chip is embedded in the black rectangle, the red band is a coil of (copper?) wires. From the chip there is one wire to the inner part of the coil, and one wire going over the coil to the outer part. The wires are to small to count the number of revolutions. There must be another coil in the door readers, which power the RFID tag through induction. The entire setup is embedded in some sort of transparent epoxy to protect it.

With a sharp knife it is possible to remove some of the plastic of the gray part.
When it is small enough, it fits in this key beautifier.
And it's finished! Shown together with the original for comparison.